In recent years, computer output devices including display screens, color printers and output devices capable of displaying a quality picture have become increasingly popular. Unfortunately, there is associated with outputting a quality picture a certain amount of image filtering or image processing. For example, image filtering may be used to enhance image detail following a deterioration of image data resulting from inputting an image using an input device and subsequently displaying the input image on an output device. Typically, blurring or defocusing of an image occurs when a picture (image) is scanned using an image scanner, or when a geometrically transformed image is constructed by means of interpolation between image picture elements.
A technique commonly adopted in photography for overcoming the problems with such defocusing or blurring of the scanned image is to sharpen the image using an "unsharp mask" filter. The unsharp mask filter takes its name from traditional photographic darkroom techniques of enhancing the edges of a graphical object of an image by removing or subtracting an "unsharp" or low-pass filtered (smoothed) from of the image from the original image.
The general principle of unsharp mask filtering is to add to an original image signal, representing an image, a constant amount of high-pass filtered signal (or "edge" sharpened signal) derived from the original image signal. The high-pass filtered signal is determined, for example, by removing from the original image signal a low-pass (unsharpened) filtered signal derived from the original image signal. Unfortunately, this traditional form of unsharp filtering is disadvantageous in that it results in an abrupt transition between the original signal and the high-pass filtered signal, especially where an abrupt edge is present in the original image. Further, conventional unsharp filtering is disadvantageous in that it also enhances high-frequency noise contained in the input image.
Thus, a need clearly exists for an improved image filtering technique that overcomes one or more of the disadvantages of conventional image "sharpening" techniques. A need exists for an image filtering technique in which an image signal and one or more filtered signals, or two or more differently filtered signals are combined in a predefined manner to reduce or eliminate noise in the output signal. A need also exists for an image sharpening technique that has a soft transition providing a smooth change between an original image signal and high-pass filtered or sharpened signal. Preferably, the transition between the original image signal and the high-pass filtered signal can be determined by a predetermined function of the original image signal, rather than filtering by a constant amount. Further, it is desirable to have an image filtering method that can be easily implemented with image compositing,